In recent years, there has been development of organic semiconducting materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, photovoltaic (PV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
Pentacene has shown promise as an organic semiconducting material. Pentacene has been described as requiring a highly crystalline structure in order to provide a molecular orientation which results in good charge mobility. Thus, in the prior art, thin films of pentacene have been vapour deposited, due in part to the fact that pentacene is rather insoluble in common solvents. However, vapour deposition requires expensive and sophisticated equipment. In view of the latter problem, one approach has been to apply a solution containing a precursor pentacene and then chemically converting, for example by heat, the precursor compound into pentacene. However, the latter method is also complex and it is difficult to control in order to obtain the necessary ordered structure for good charge mobility.
Soluble pentacene compounds have recently been described in the prior art as organic semiconducting compounds, see for example US 2003/0116755 A and U.S. Pat. No. 6,690,029. The use of pentacenes in FETs has been suggested in WO 03/016599, in which a solution of a soluble pentacene was deposited on a substrate and the solvent evaporated to form a thin film of the pentacene. However, soluble pentacenes have been described in U.S. Pat. No. 6,690,029 and WO 03/016599 as still requiring a highly crystalline structure in the thin film for acceptable charge mobility, especially when used in FETs, which means that the pentacenes must still be deposited in a controlled way. Thus, the prior art is careful not to dilute the pentacene in any way, otherwise it would be expected to disrupt the crystalline structure of the pentacene and hence reduce charge mobility.
Improved charge mobility is one goal of new electronic devices. Another goal is improved stability, film uniformity and integrity of the organic semiconductor layer. One way potentially to improve organic semiconductor layer stability and integrity in devices would be to include the organic semiconducting component in an organic binder. However, whenever an organic semiconducting component is combined with a binder it is effectively “diluted” by the binder and a reduction of charge mobility is to be expected. Among other things, diluting an organic semiconductor by mixing with binders disrupts the molecular order in the semiconducting layer. Diluting an organic semiconducting component in the channel of an OFET for example is particularly problematic as any disruption of the orbital overlap between molecules in the immediate vicinity of the gate insulator (the first few molecular layers) is expected to reduce mobility. Electrons or holes are then forced to extend their path into the bulk of the organic semiconductor, which is undesirable. Certain organic semiconducting materials are expected to be more susceptible than others to the effects of use in a binder. Since pentacenes have been taught as requiring highly ordered structures for useful charge mobility, it has not previously been considered desirable to include pentacenes with binders. In WO 03/030278 it was attempted to use binders but there it was shown that a gradual reduction of FET mobility occurs when a (precursor) pentacene is mixed with increasing amounts of binder, even with amounts of less than 5% binder.
Certain low polarity binder resins are described in WO 02/45184 for use with organic semiconductors in FETs. However, a reduction in charge mobility is still expected when the semiconductor is diluted in the binder.
WO 2005/055248 A2 relates to a semiconductor formulation comprising an organic binder and a polyacene, but does not explicitly disclose the materials claimed in the present invention.
One aim of the present invention is to reduce or overcome the disadvantages in organic semiconducting layers as described above. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It was now found that these aims can be achieved by providing semiconducting materials, formulations and methods as claimed in the present invention. Especially, it was found that, by providing unsymmetric polyacenes as claimed in the present invention, charge transport and semiconducting materials with improved solubility, charge carrier mobility and stability can be obtained. Furthermore it was found that, when these unsymmetric polyacenes are provided in a formulation together with an organic binder, improved semiconducting materials with good processability are obtained which do still show a surprisingly high charge carrier mobility.